Total joint replacements are appliances which are surgically implanted to treat advanced stages of joint disease. Generally these appliances are articulating prosthetic devices which in today's medical field are commonly used to replace various joints such as damaged or diseased hips, knees, elbows, wrists and fingers. Most prosthetic joint designs consist of two principal components: (1) a concave socket component; and (2) a generally ball-shaped head which seats in the socket. The socket is usually made from a polymeric material having suitable frictional and biological characteristics such as ultra high molecular weight polyethylene (UHMWPE). The head component articulates in the socket and is usually a suitable metallic material such as stainless steel, titanium or other alloys. The two components are attached or implanted at the articulating bones by a surgical procedure replacing the diseased or damaged joint. The resulting artificial prosthetic joint articulates in a manner closely similar to that of the natural joint. In some cases a partial joint placement occurs and only one component is implanted.
One of the most common procedures of this type is the total hip arthroplasty commonly termed the total hip replacement (THR). The THR procedure involves the implantation of a polymeric cup in the acetabulum. The other component is a spherical ball which is carried on an elongate stem. The ball and stem are typically fabricated from a suitable metal such as stainless steel, titanium or other biologically-acceptable alloys. The stem is implanted in the medullary canal of the proximal end of the femur. The spherical ball fits snugly in the polymeric cup creating an articulating joint.
Most joint replacement components require a very close fit with the bone in order to achieve firm anchorage. This is commonly accomplished by applying bone cement or injecting bone cement into the medullary canal prior to inserting the appliance into the bone. More recently, various porous coatings have been developed to which the bone attaches directly eliminating the need for cement. In either case, the interposition of soft tissue between the implant or cement mantle and the bone will result in compromised fixation of the prosthesis.
Loosening of one or both components of the implants is a major complication associated with prosthetic joint replacements. It is also generally recognized that the debris associated with total joint replacements often cause localized or linear osteolysis resulting in the loosening of the prosthesis. Debris from total hip arthroplasty devices generally fall into three basic categories; polyethylene debris from the acetabular component, polymethylmethacrylate (PMMA) debris associated with implants that have been inserted with cement and metal debris. Polymeric and metallic wear debris results from abrasion as the joint articulates and over time this debris begins to accumulate at the proximal end of the femur. The wear rate may be accelerated if debris from the femoral implant site, for example bone cement particles, migrate upwardly to the articulating surfaces causing third-body abrasion. Large quantities of wear debris accumulating in the soft tissue at the proximal end of the femur cause an inflammatory reaction. The reactive soft tissue can cause resorption of the bone at the proximal end of the femur, especially when debris invades the implant-bone interface. The process accelerates as the debris-laden tissue works its way down the stem of the femoral component into medullary canal. This effect is mediated by the metal itself or by the corrosion or fretting products released by certain metals or metal alloys. The migration of debris is believed due in part to joint fluid which carries the debris both upwardly to the interface of the socket and the ball and downwardly along the stem. Joint fluid penetrates far more extensively than previously believed even in a well-fixed joint component.
There are a number of recent articles in the medical literature dealing with wear debris induced osteolysis. As for example see: Periprosthetic Bone Loss in Total Hip Arthroplasty, The Journal of Bone and Joint Surgery, Vol. 74-A, No. 6, July 1992; The Problem in Total Joint Arthroplasty: Aseptic Loosening, The Journal of Bone and Joint Surgery, Vol. 75-A, No. 6, June 1993; Mechanism and Clinical Significance of Wear Debris-Induced Osteolysis, Clinical Orthopaedics and Related Research, No. 276, March, 1992.
Recent findings (Isolation and Characterization of Debris in Membranes around Total Joint Prostheses, The Journal of Bone and Joint Surgery, Vol. 76-A, No. 11, November 1994; Composition and Morphology of Wear Debris in Failed Uncemented Total Hip Replacement, The Journal of Bone and Joint Surgery, Vol. 76-B, No. 1, January 1994 ) suggest that the average wear particle diameter associated with THR is between 0.5 and 0.8 microns with particles ranging down to 0.2 microns in diameter.
Various suggestions for dealing with the problem of wear debris induced osteolysis can be found in the prior art. There has been major emphasis on improving cementing techniques or using alternate fixation methods. Other writers suggest reduction of wear debris should be the primary goal of orthopedic research in the future. Membranous enclosures have also been proposed for use with articulate hip joint replacements which isolate or encapsulate the joint. U.S. Pat. No. 3,683,421 discloses a total hip replacement device that comprises a membrane enclosure attached to both components of the joint replacement encapsulating the articulating portion of the joint replacement thus isolating it from the body. The enclosure is filled with a synthetic lubricant material.
U.S. Pat. No. 3,739,403 discloses a joint replacement comprising an encapsulating membranous enclosure. In this case, an enclosure capsule is provided to prevent tissue from growing into the articulating portion of the joint replacement. The enclosure may have small holes which allow body fluid to pass through the capsule providing joint lubrication.
Thus, while the various approaches suggested above may also serve to limit migration of debris and tissue, several problems are associated with an enclosure which extends around the articulating portion of a joint replacement and which is attached to both components. An enclosure or capsule attached to both components of an articulating joint replacement will decrease flexibility of the joint. Continued long-term flexure of the joint will also limit useful life of the capsule due to fatigue failure of the capsule membrane. Preventing dispersion of wear debris away from the articulating components will increase third-body abrasion leading to earlier failure of one or both components. Further, a capsule will not prevent adjacent soft tissue from migrating to and into bone-implant interfaces. Thus, in view of the foregoing it is apparent that there exists a significant problem of complications as a result of wear debris induced osteolysis. While various approaches can be found or are suggested in the medical and patent literature, these approaches have not found wide acceptance in the medical community.